1. (13.4) Magnetic Field of a Coil or Solenoid

2. Electromagnet: Solenoid: - acts like a magnet when an electric current
- object that exerts a magnetic force using electricity
Solenoid:
- a coil of wire
- acts like a magnet when an electric current
passes through it
- produces a magnetic field like the magnetic
field of a bar magnet.

3. Uses of Electromagnets
Usually for moving things or storing information
In your computer
- hard disk drive
In the junk yard

4. Other Applications There are many Some are simple - doorbell
Some are advanced
- loudspeaker
- MRI
- TV
- cell phone
- telephone

5. Direction of Magnetic Field - Right-Hand Rule for a Coil
If a coil is grasped in the right hand with the curled fingers representing the direction of current, the thumb points in the direction of the magnetic field inside the coil.
p483

6. How Does That Look?
p483

7. Let’s Practice! p. 483 #1 Indicate the direction of electric current
magnetic field lines
N and S poles of coil

8. Let’s Practice! p. 488 #4 a and b Indicate the direction of
electric current
magnetic field lines
N and S poles of coil

9. Let’s Practice! p. 489 #1 a and b Indicate N and S poles of coil
direction of compass needle (compass is shown as empty circle)

10. Strength of the Magnetic Field
There are several factors…
Current in the coil
F  I
Number of loops (or turns) in the coil
F  N
Permeability of the Core
(i.e. the factor by which a core material increases the magnetic field strength)

11. (13.5) The Motor Principle

12. The Motor Principle
Consider a conductor that carries a current and cuts through an existing magnetic field
Moving charges in an electric current experience a force due to magnetic field.

13. Force on a Current Carrying Wire in a Magnetic Field

14. Right-Hand Rule for the Motor Principle
If the fingers of the open right hand point in the direction of the external magnetic field, and the thumb represents the direction of the current, the force on the conductor will be in the direction of the palm.
p491

15. Let’s Practice! p493 #1. Use Figure 7 to do the following:
(a) Draw the magnetic fields of the permanent magnet and the conductor.
(b) Determine the direction of the force on the conductor.

16. Let’s Practice!
p502
#3. The conductors shown in Figure 16 represent a loop in a magnetic field.
Determine whether the force on the loop is clockwise or counter clockwise. Use a diagram to explain your answer.

17. Let’s Practice!
p502
# 4. For the instant shown in Figure 17, is the force on the loop clockwise
or counter clockwise? Explain your reasoning.

18. DC MOTOR - An Application of the Motor Principle
13.6
DC MOTOR
- An Application of the Motor Principle

19. The Electric Motor Motors can be used in small battery operated toys.
Subway trains and diesel locomotives use large-scale motors.

20. The Electric Motor Recall:
According to the motor principle, a current-carrying conductor in an external magnetic field will experience a force.
The force exerted on a current carrying coil will cause the coil to rotate.
external magnet
coil

21. The Electric Motor – The Parts
Field Magnet
- this is a permanent magnet
Armature Coil
able to rotate in the magnetic field
core of the helix
can be magnetized
Split ring commutator
transfers current from brushes to armature
allows current to flow in and out of the coil even when it is rotating.
Brushes
- made of graphite (good conductor and a lubricant)
- allows current to form an external circuit through the commutator to the loop

22. Electric Motor – Let’s See That !
Follow the Link
DC Motor Java Applet

23. The Electric Motor
Another Illustration (p496 Figure 5) Consider a current-carrying coil in an external magnetic field.

24. The Electric Motor
p496 Figure 6

25. The Electric Motor What would make the motor turn more quickly?
More Force is produced by
More current through armature
More turns in the armature
Stronger magnetic field

26. How is it Done?
Follow these steps and you can make a DC motor yourself! How To Build a Simple DC Motor

27. Some Applications - Speakers
Loudspeaker has a membrane but oscillations are created by variations in electrical current, which cause an electromagnet to be pulled towards and away from a second, permanent magnet.
These oscillations cause the membrane of the loudspeaker to vibrate with the same frequency as the oscillations in the electrical current.
Headphones work essentially the same way, they’re just smaller.

28. Magnets & Computers– Don’t Mix !
Warning: Strong magnets can permanently damage a computer hard drive.